A relay is an electrically operated switch . It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms , such as make contacts, break contacts, or combinations thereof.
112-477: Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. Relays were first used in long-distance telegraph circuits as signal repeaters: they refresh the signal coming in from one circuit by transmitting it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations. The traditional electromechanical form of
224-401: A form factor that allows compactly installing many relays in a control panel. Although such relays once were the backbone of automation in such industries as automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications. A relay allows circuits to be switched by electrical equipment: for example, a timer circuit with
336-477: A thermocouple or resistance thermometer sensor embedded in the winding. A polarized relay places the armature between the poles of a permanent magnet to increase sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and correct telegraphic distortion . A reed relay is a reed switch enclosed in a solenoid. The switch has a set of contacts inside an evacuated or inert gas -filled glass tube that protects
448-469: A voltage spike dangerous to semiconductor circuit components. Such diodes were not widely used before the application of transistors as relay drivers, but soon became ubiquitous as early germanium transistors were easily destroyed by this surge. Some automotive relays include a diode inside the relay case. Resistors, while more durable than diodes, are less efficient at eliminating voltage spikes generated by relays and therefore not as commonly used. If
560-506: A 175-yard (160 m) long trench as well as an eight-mile (13 km) long overhead telegraph. The lines were connected at both ends to revolving dials marked with the letters of the alphabet and electrical impulses sent along the wire were used to transmit messages. Offering his invention to the Admiralty in July 1816, it was rejected as "wholly unnecessary". His account of the scheme and
672-480: A clock-face, and the signal caused a needle to indicate the letter. This early system required the receiver to be present in real time to record the message and it reached speeds of up to 15 words a minute. In 1846, Alexander Bain patented a chemical telegraph in Edinburgh. The signal current moved an iron pen across a moving paper tape soaked in a mixture of ammonium nitrate and potassium ferrocyanide, decomposing
784-489: A convenient means of generating fast rise time pulses, however although the rise time may be picoseconds, the exact timing of the event is, like all other types of relay, subject to considerable jitter, possibly milliseconds, due to mechanical variations. The same coalescence process causes another effect, which is a nuisance in some applications. The contact resistance is not stable immediately after contact closure, and drifts, mostly downwards, for several seconds after closure,
896-411: A feedback loop or sequential circuit . Such an electrically latching relay requires continuous power to maintain state, unlike magnetically latching relays or mechanically ratcheting relays. While (self-)holding circuits are often realized with relays they can also be implemented by other means. In computer memories, latching relays and other relays were replaced by delay-line memory , which in turn
1008-468: A few kilometers (in von Sömmering's design), with each of the telegraph receiver's wires immersed in a separate glass tube of acid. An electric current was sequentially applied by the sender through the various wires representing each letter of a message; at the recipient's end, the currents electrolysed the acid in the tubes in sequence, releasing streams of hydrogen bubbles next to each associated letter or numeral. The telegraph receiver's operator would watch
1120-405: A fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity
1232-435: A good conductor. Contactors with overload protection devices are often used to start motors. A force-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of force-guided contacts
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#17327917443181344-412: A motor, one can perform this simple exercise: with an incandescent light on, cause a large motor such as a drill press, saw, air conditioner compressor, or vacuum cleaner to start. The light may dim briefly as the motor starts. When the armature is not turning (called locked rotor) there is no back-EMF and the motor's current draw is quite high. If the motor's starting current is high enough, it will pull
1456-698: A moving paper tape by a stylus which was operated by an electromagnet. Morse and Vail developed the Morse code signalling alphabet . On May 24, 1844, Morse sent to Vail the historic first message “ WHAT HATH GOD WROUGHT " from the Capitol in Washington to the old Mt. Clare Depot in Baltimore . The first commercial electrical telegraph was the Cooke and Wheatstone system . A demonstration four-needle system
1568-576: A permanent magnet and connecting the coil with the transmission wires by means of the commutator. The page of Gauss's laboratory notebook containing both his code and the first message transmitted, as well as a replica of the telegraph made in the 1850s under the instructions of Weber are kept in the faculty of physics at the University of Göttingen , in Germany. Gauss was convinced that this communication would be of help to his kingdom's towns. Later in
1680-426: A practical alphabetical system in 1840 called the A.B.C. System, used mostly on private wires. This consisted of a "communicator" at the sending end and an "indicator" at the receiving end. The communicator consisted of a circular dial with a pointer and the 26 letters of the alphabet (and four punctuation marks) around its circumference. Against each letter was a key that could be pressed. A transmission would begin with
1792-458: A relay could switch power at a preset time. For many years relays were the standard method of controlling industrial electronic systems. A number of relays could be used together to carry out complex functions ( relay logic ). The principle of relay logic is based on relays which energize and de-energize associated contacts. Relay logic is the predecessor of ladder logic , which is commonly used in programmable logic controllers . A mercury relay
1904-574: A relay uses an electromagnet to close or open the contacts, but relays using other operating principles have also been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts . Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called protective relays or safety relays . Latching relays require only
2016-447: A relay with several normally closed (NC) contacts may stick to the unenergized position, so that when energized, the circuit through one set of contacts is broken, with a marginal gap, while the other remains closed. By introducing both NO and NC contacts, or more commonly, changeover contacts, on the same relay, it then becomes possible to guarantee that if any NC contact is closed, all NO contacts are open, and conversely, if any NO contact
2128-681: A safety function is to use appropriate measures to reduce the existing risk to an acceptable level. A solid-state contactor is a heavy-duty solid state relay, including the necessary heat sink, used where frequent on-off cycles are required, such as with electric heaters, small electric motors , and lighting loads. There are no moving parts to wear out and there is no contact bounce due to vibration. They are activated by AC control signals or DC control signals from programmable logic controllers (PLCs), PCs, transistor-transistor logic (TTL) sources, or other microprocessor and microcontroller controls. Electrical telegraph Electrical telegraphy
2240-545: A sensitive indicator for an electric current. Also that year, André-Marie Ampère suggested that telegraphy could be achieved by placing small magnets under the ends of a set of wires, one pair of wires for each letter of the alphabet. He was apparently unaware of Schweigger's invention at the time, which would have made his system much more sensitive. In 1825, Peter Barlow tried Ampère's idea but only got it to work over 200 feet (61 m) and declared it impractical. In 1830 William Ritchie improved on Ampère's design by placing
2352-601: A short-distance transmission of signals between two telegraphs in different rooms of his apartment. In 1836, the British government attempted to buy the design but Schilling instead accepted overtures from Nicholas I of Russia . Schilling's telegraph was tested on a 5-kilometre-long (3.1 mi) experimental underground and underwater cable, laid around the building of the main Admiralty in Saint Petersburg and
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#17327917443182464-475: A single pulse of control power to operate the switch persistently. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effects. Magnetic latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling. Electrical relays got their start in application to telegraphs . American scientist Joseph Henry
2576-405: A steady rhythm, and the usual speed of operation was 30 words per minute. By this point, reception had been automated, but the speed and accuracy of the transmission were still limited to the skill of the human operator. The first practical automated system was patented by Charles Wheatstone. The message (in Morse code ) was typed onto a piece of perforated tape using a keyboard-like device called
2688-467: A telegraph along the Nuremberg–Fürth railway line , built in 1835 as the first German railroad, which was the first earth-return telegraph put into service. By 1837, William Fothergill Cooke and Charles Wheatstone had co-developed a telegraph system which used a number of needles on a board that could be moved to point to letters of the alphabet. Any number of needles could be used, depending on
2800-404: A thin, self-renewing film of liquid mercury. For higher-power relays switching many amperes, such as motor circuit contactors, contacts are made with a mixtures of silver and cadmium oxide, providing low contact resistance and high resistance to the heat of arcing. Contacts used in circuits carrying scores or hundreds of amperes may include additional structures for heat dissipation and management of
2912-417: A typical EN 50005-compliant SPDT relay's terminals would be numbered 11, 12, 14, A1 and A2 for the C, NC, NO, and coil connections, respectively. DIN 72552 defines contact numbers in relays for automotive use: Where radio transmitters and receivers share one antenna, often a coaxial relay is used as a TR (transmit-receive) relay, which switches the antenna from the receiver to the transmitter. This protects
3024-477: A useful communication system. In 1774, Georges-Louis Le Sage realised an early electric telegraph. The telegraph had a separate wire for each of the 26 letters of the alphabet and its range was only between two rooms of his home. In 1800, Alessandro Volta invented the voltaic pile , providing a continuous current of electricity for experimentation. This became a source of a low-voltage current that could be used to produce more distinct effects, and which
3136-696: Is a point-to-point text messaging system, primarily used from the 1840s until the late 20th century. It was the first electrical telecommunications system and the most widely used of a number of early messaging systems called telegraphs , that were devised to send text messages more quickly than physically carrying them. Electrical telegraphy can be considered the first example of electrical engineering . Text telegraphy consisted of two or more geographically separated stations, called telegraph offices . The offices were connected by wires, usually supported overhead on utility poles . Many electrical telegraph systems were invented that operated in different ways, but
3248-435: Is a relay that uses mercury as the switching element. They are used where contact erosion would be a problem for conventional relay contacts. Owing to environmental considerations about significant amount of mercury used and modern alternatives, they are now comparatively uncommon. A mercury-wetted reed relay is a form of reed relay that employs a mercury switch , in which the contacts are wetted with mercury . Mercury reduces
3360-457: Is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing . When the coil is energized with direct current , a flyback diode or snubber resistor is often placed across the coil to dissipate the energy from the collapsing magnetic field ( back EMF ) at deactivation, which would otherwise generate
3472-630: Is closed, all NC contacts are open. It is not possible to reliably ensure that any particular contact is closed, except by potentially intrusive and safety-degrading sensing of its circuit conditions, however in safety systems it is usually the NO state that is most important, and as explained above, this is reliably verifiable by detecting the closure of a contact of opposite sense. Force-guided contact relays are made with different main contact sets, either NO, NC or changeover, and one or more auxiliary contact sets, often of reduced current or voltage rating, used for
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3584-412: Is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of
3696-534: Is needed. A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges . An earth-leakage circuit breaker includes a specialized latching relay. Very early computers often stored bits in a magnetically latching relay, such as ferreed or the later remreed in the 1ESS switch . Some early computers used ordinary relays as a kind of latch —they store bits in ordinary wire-spring relays or reed relays by feeding an output wire back as an input, resulting in
3808-437: Is often cited to have invented a relay in 1835 in order to improve his version of the electrical telegraph , developed earlier in 1831. However, an official patent was not issued until 1840 to Samuel Morse for his telegraph, which is now called a relay. The mechanism described acted as a digital amplifier, repeating the telegraph signal, and thus allowing signals to be propagated as far as desired. The word relay appears in
3920-406: Is partly offset by the increased costs in the external circuit. In another type, a ratchet relay has a ratchet mechanism that holds the contacts closed after the coil is momentarily energized. A second impulse, in the same or a separate coil, releases the contacts. This type may be found in certain cars, for headlamp dipping and other functions where alternating operation on each switch actuation
4032-399: Is present; changing the orientation of the reeds or degaussing the switch with respect to the solenoid's magnetic field can resolve this problem. Sealed contacts with mercury-wetted contacts have longer operating lives and less contact chatter than any other kind of relay. Safety relays are devices which generally implement protection functions. In the event of a hazard, the task of such
4144-433: Is relative motion between the armature and the magnetic field produced by the motor's field coils or permanent magnet field, thus also acting as a generator while running as a motor. This effect is not due to the motor's inductance, which generates a voltage in opposition to a changing current via Faraday's law, but a separate phenomenon. That is, the back-EMF is also due to inductance and Faraday's law, but occurs even when
4256-438: Is to enable the safety circuit to check the status of the relay. Force-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", "mechanically linked contacts", or "safety relays". These safety relays have to follow design rules and manufacturing rules that are defined in one main machinery standard EN 50205 : Relays with forcibly guided (mechanically linked) contacts. These rules for
4368-399: Is to indirectly measure motor speed and position, as the back-EMF is proportional to the rotational speed of the armature. In motor control and robotics, back-EMF often refers most specifically to actually using the voltage generated by a spinning motor to infer the speed of the motor's rotation, for use in better controlling the motor in specific ways. To observe the effect of back-EMF of
4480-520: Is used to split the flux into two out-of-phase components which add together, increasing the minimum pull on the armature during the AC cycle. Typically this is done with a small copper "shading ring" crimped around a portion of the core that creates the delayed, out-of-phase component, which holds the contacts during the zero crossings of the control voltage. Contact materials for relays vary by application. Materials with low contact resistance may be oxidized by
4592-470: The 'Stick Punch'. The transmitter automatically ran the tape through and transmitted the message at the then exceptionally high speed of 70 words per minute. An early successful teleprinter was invented by Frederick G. Creed . In Glasgow he created his first keyboard perforator, which used compressed air to punch the holes. He also created a reperforator (receiving perforator) and a printer. The reperforator punched incoming Morse signals onto paper tape and
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4704-579: The ITA-1 Baudot code , a five-bit code. This yielded only thirty-two codes, so it was over-defined into two "shifts", "letters" and "figures". An explicit, unshared shift code prefaced each set of letters and figures. In 1901, Baudot's code was modified by Donald Murray . In the 1930s, teleprinters were produced by Teletype in the US, Creed in Britain and Siemens in Germany. By 1935, message routing
4816-591: The Imperial palace at Tsarskoye Selo and Kronstadt Naval Base . In 1833, Carl Friedrich Gauss , together with the physics professor Wilhelm Weber in Göttingen , installed a 1,200-metre-long (3,900 ft) wire above the town's roofs. Gauss combined the Poggendorff-Schweigger multiplicator with his magnetometer to build a more sensitive device, the galvanometer . To change the direction of
4928-459: The Morse system became the standard for international communication, using a modified form of Morse's code that had been developed for German railways. Electrical telegraphs were used by the emerging railway companies to provide signals for train control systems, minimizing the chances of trains colliding with each other. This was built around the signalling block system in which signal boxes along
5040-519: The Morse system. As well as the rapid expansion of the use of the telegraphs along the railways, they soon spread into the field of mass communication with the instruments being installed in post offices . The era of mass personal communication had begun. Telegraph networks were expensive to build, but financing was readily available, especially from London bankers. By 1852, National systems were in operation in major countries: The New York and Mississippi Valley Printing Telegraph Company, for example,
5152-550: The Relay and Switch Industry Association define 23 distinct electrical contact forms found in relays and switches. Of these, the following are commonly encountered: The S ( single ) or D ( double ) designator for the pole count may be replaced with a number, indicating multiple contacts connected to a single actuator . For example, 4PDT indicates a four-pole double-throw relay that has 12 switching terminals. EN 50005 are among applicable standards for relay terminal numbering;
5264-539: The air, or may tend to "stick" instead of cleanly parting when opening. Contact material may be optimized for low electrical resistance, high strength to withstand repeated operations, or high capacity to withstand the heat of an arc. Where very low resistance is required, or low thermally-induced voltages are desired, gold-plated contacts may be used, along with palladium and other non-oxidizing, semi-precious metals. Silver or silver-plated contacts are used for signal switching. Mercury-wetted relays make and break circuits using
5376-582: The alphabet, a message could be transmitted by connecting the wire terminals in turn to an electrostatic machine, and observing the deflection of pith balls at the far end. The writer has never been positively identified, but the letter was signed C.M. and posted from Renfrew leading to a Charles Marshall of Renfrew being suggested. Telegraphs employing electrostatic attraction were the basis of early experiments in electrical telegraphy in Europe, but were abandoned as being impractical and were never developed into
5488-546: The application of electricity to communications at a distance. All the known effects of electricity – such as sparks , electrostatic attraction , chemical changes , electric shocks , and later electromagnetism – were applied to the problems of detecting controlled transmissions of electricity at various distances. In 1753, an anonymous writer in the Scots Magazine suggested an electrostatic telegraph. Using one wire for each letter of
5600-461: The arc produced when interrupting the circuit. Some relays have field-replaceable contacts, such as certain machine tool relays; these may be replaced when worn out, or changed between normally open and normally closed state, to allow for changes in the controlled circuit. Since relays are switches , the terminology applied to switches is also applied to relays; a relay switches one or more poles , each of whose contacts can be thrown by energizing
5712-553: The armature was intended to make marks on paper tape, but operators learned to interpret the clicks and it was more efficient to write down the message directly. In 1851, a conference in Vienna of countries in the German-Austrian Telegraph Union (which included many central European countries) adopted the Morse telegraph as the system for international communications. The international Morse code adopted
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#17327917443185824-420: The bar, creating a much more powerful electromagnet which could operate a telegraph through the high resistance of long telegraph wires. During his tenure at The Albany Academy from 1826 to 1832, Henry first demonstrated the theory of the 'magnetic telegraph' by ringing a bell through one-mile (1.6 km) of wire strung around the room in 1831. In 1835, Joseph Henry and Edward Davy independently invented
5936-403: The bubbles and could then record the transmitted message. This is in contrast to later telegraphs that used a single wire (with ground return). Hans Christian Ørsted discovered in 1820 that an electric current produces a magnetic field that will deflect a compass needle. In the same year Johann Schweigger invented the galvanometer , with a coil of wire around a compass, that could be used as
6048-457: The change perhaps being 0.5 ohm. Multi-voltage relays are devices designed to work for wide voltage ranges such as 24 to 240 VAC and VDC and wide frequency ranges such as 0 to 300 Hz. They are indicated for use in installations that do not have stable supply voltages. Electric motors need overcurrent protection to prevent damage from over-loading the motor, or to protect against short circuits in connecting cables or internal faults in
6160-697: The chemical and producing readable blue marks in Morse code. The speed of the printing telegraph was 16 and a half words per minute, but messages still required translation into English by live copyists. Chemical telegraphy came to an end in the US in 1851, when the Morse group defeated the Bain patent in the US District Court. For a brief period, starting with the New York–Boston line in 1848, some telegraph networks began to employ sound operators, who were trained to understand Morse code aurally. Gradually,
6272-452: The circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke , which is soldered to the PCB. When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with
6384-421: The coil. Normally open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. Normally closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. All of the contact forms involve combinations of NO and NC connections. The National Association of Relay Manufacturers and its successor,
6496-405: The coil. The advantage is that one coil consumes power only for an instant while the relay is being switched, and the relay contacts retain this setting across a power outage. A latching relay allows remote control of building lighting without the hum that may be produced from a continuously (AC) energized coil. In one mechanism, two opposing coils with an over-center spring or permanent magnet hold
6608-447: The communicator. Pressing another key would then release the pointer and the previous key, and re-connect the magneto to the line. These machines were very robust and simple to operate, and they stayed in use in Britain until well into the 20th century. The Morse system uses a single wire between offices. At the sending station, an operator taps on a switch called a telegraph key , spelling out text messages in Morse code . Originally,
6720-525: The contact resistance and mitigates the associated voltage drop. Surface contamination may result in poor conductivity for low-current signals. For high-speed applications, the mercury eliminates contact bounce, and provides virtually instantaneous circuit closure. Mercury wetted relays are position-sensitive and must be mounted according to the manufacturer's specifications. Because of the toxicity and expense of liquid mercury, these relays have increasingly fallen into disuse. The high speed of switching action of
6832-479: The contacts against atmospheric corrosion ; the contacts are made of magnetic material that makes them move under the influence of the field of the enclosing solenoid or an external magnet. Reed relays can switch faster than larger relays and require very little power from the control circuit. However, they have relatively low switching current and voltage ratings. Though rare, the reeds can become magnetized over time, which makes them stick "on", even when no current
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#17327917443186944-411: The contacts in position after the coil is de-energized. A pulse to one coil turns the relay on, and a pulse to the opposite coil turns the relay off. This type is widely used where control is from simple switches or single-ended outputs of a control system, and such relays are found in avionics and numerous industrial applications. Another latching type has a remanent core that retains the contacts in
7056-545: The contacts. To prevent short over current spikes from causing nuisance triggering the armature movement is damped with a dashpot . The thermal and magnetic overload detections are typically used together in a motor protection relay. Electronic overload protection relays measure motor current and can estimate motor winding temperature using a "thermal model" of the motor armature system that can be set to provide more accurate motor protection. Some motor protection relays include temperature detector inputs for direct measurement from
7168-459: The context of electromagnetic operations from 1860 onwards. A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core (a solenoid), an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature , and one or more sets of contacts (there are two contacts in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. The armature
7280-591: The east coast by 24 October 1861, bringing an end to the Pony Express . France was slow to adopt the electrical telegraph, because of the extensive optical telegraph system built during the Napoleonic era . There was also serious concern that an electrical telegraph could be quickly put out of action by enemy saboteurs, something that was much more difficult to do with optical telegraphs which had no exposed hardware between stations. The Foy-Breguet telegraph
7392-504: The electric current, he constructed a commutator of his own. As a result, he was able to make the distant needle move in the direction set by the commutator on the other end of the line. At first, Gauss and Weber used the telegraph to coordinate time, but soon they developed other signals and finally, their own alphabet. The alphabet was encoded in a binary code that was transmitted by positive or negative voltage pulses which were generated by means of moving an induction coil up and down over
7504-605: The electric telegraph, visual systems were used, including beacons , smoke signals , flag semaphore , and optical telegraphs for visual signals to communicate over distances of land. An auditory predecessor was West African talking drums . In the 19th century, Yoruba drummers used talking drums to mimic human tonal language to communicate complex messages – usually regarding news of birth, ceremonies, and military conflict – over 4–5 mile distances. From early studies of electricity , electrical phenomena were known to travel with great speed, and many experimenters worked on
7616-521: The first means of radiowave telecommunication, which he began in 1894. In the early 20th century, manual operation of telegraph machines was slowly replaced by teleprinter networks. Increasing use of the telephone pushed telegraphy into only a few specialist uses; its use by the general public dwindled to greetings for special occasions. The rise of the Internet and email in the 1990s largely made dedicated telegraphy networks obsolete. Prior to
7728-453: The line communicate with neighbouring boxes by telegraphic sounding of single-stroke bells and three-position needle telegraph instruments. In the 1840s, the electrical telegraph superseded optical telegraph systems such as semaphores, becoming the standard way to send urgent messages. By the latter half of the century, most developed nations had commercial telegraph networks with local telegraph offices in most cities and towns, allowing
7840-491: The magnetic needles inside a coil of wire connected to each pair of conductors. He successfully demonstrated it, showing the feasibility of the electromagnetic telegraph, but only within a lecture hall. In 1825, William Sturgeon invented the electromagnet , with a single winding of uninsulated wire on a piece of varnished iron , which increased the magnetic force produced by electric current. Joseph Henry improved it in 1828 by placing several windings of insulated wire around
7952-477: The mercury dipping electrical relay , in which a magnetic needle is dipped into a pot of mercury when an electric current passes through the surrounding coil. In 1837, Davy invented the much more practical metallic make-and-break relay which became the relay of choice in telegraph systems and a key component for periodically renewing weak signals. Davy demonstrated his telegraph system in Regent's Park in 1837 and
8064-406: The mercury-wetted relay is a notable advantage. The mercury globules on each contact coalesce , and the current rise time through the contacts is generally considered to be a few picoseconds. However, in a practical circuit it may be limited by the inductance of the contacts and wiring. It was quite common, before restrictions on the use of mercury, to use a mercury-wetted relay in the laboratory as
8176-456: The monitoring system. Contacts may be all NO, all NC, changeover, or a mixture of these, for the monitoring contacts, so that the safety system designer can select the correct configuration for the particular application. Safety relays are used as part of an engineered safety system. A latching relay, also called impulse , bistable , keep , or stay relay, or simply latch , maintains either contact position indefinitely without power applied to
8288-432: The most widely used of its type was the Cooke and Wheatstone telegraph , invented in 1837. The second category are armature systems, in which the current activates a telegraph sounder that makes a click; communication on this type of system relies on sending clicks in coded rhythmic patterns. The archetype of this category was the Morse system and the code associated with it, both invented by Samuel Morse in 1838. In 1865,
8400-448: The motor current is not changing, and arises from the geometric considerations of an armature spinning in a magnetic field. This voltage is in series with and opposes the original applied voltage and is called "back-electromotive force" (by Lenz's law ). With a lower overall voltage across the motor's internal resistance as the motor turns faster, the current flowing into the motor decreases. One practical application of this phenomenon
8512-413: The motor windings. The overload sensing devices are a form of heat operated relay where a coil heats a bimetallic strip , or where a solder pot melts, to operate auxiliary contacts. These auxiliary contacts are in series with the motor's contactor coil, so they turn off the motor when it overheats. This thermal protection operates relatively slowly allowing the motor to draw higher starting currents before
8624-422: The number of characters it was required to code. In May 1837 they patented their system. The patent recommended five needles, which coded twenty of the alphabet's 26 letters. Samuel Morse independently developed and patented a recording electric telegraph in 1837. Morse's assistant Alfred Vail developed an instrument that was called the register for recording the received messages. It embossed dots and dashes on
8736-414: The ones that became widespread fit into two broad categories. First are the needle telegraphs, in which electric current sent down the telegraph line produces electromagnetic force to move a needle-shaped pointer into position over a printed list. Early needle telegraph models used multiple needles, thus requiring multiple wires to be installed between stations. The first commercial needle telegraph system and
8848-526: The operated position by the remanent magnetism in the core. This type requires a current pulse of opposite polarity to release the contacts. A variation uses a permanent magnet that produces part of the force required to close the contact; the coil supplies sufficient force to move the contact open or closed by aiding or opposing the field of the permanent magnet. A polarity controlled relay needs changeover switches or an H-bridge drive circuit to control it. The relay may be less expensive than other types, but this
8960-415: The pointer reached the position of the depressed key, it would stop and the magneto would be disconnected from the line. The communicator's pointer was geared to the magneto mechanism. The indicator's pointer was moved by a polarised electromagnet whose armature was coupled to it through an escapement . Thus the alternating line voltage moved the indicator's pointer on to the position of the depressed key on
9072-410: The pointers on the dials at both ends set to the start position. The transmitting operator would then press down the key corresponding to the letter to be transmitted. In the base of the communicator was a magneto actuated by a handle on the front. This would be turned to apply an alternating voltage to the line. Each half cycle of the current would advance the pointers at both ends by one position. When
9184-513: The possibilities of rapid global communication in Descriptions of an Electrical Telegraph and of some other Electrical Apparatus was the first published work on electric telegraphy and even described the risk of signal retardation due to induction. Elements of Ronalds' design were utilised in the subsequent commercialisation of the telegraph over 20 years later. The Schilling telegraph , invented by Baron Schilling von Canstatt in 1832,
9296-580: The printer decoded this tape to produce alphanumeric characters on plain paper. This was the origin of the Creed High Speed Automatic Printing System, which could run at an unprecedented 200 words per minute. His system was adopted by the Daily Mail for daily transmission of the newspaper contents. With the invention of the teletypewriter , telegraphic encoding became fully automated. Early teletypewriters used
9408-417: The protection relay will trip. Where the overload relay is exposed to the same ambient temperature as the motor, a useful though crude compensation for motor ambient temperature is provided. The other common overload protection system uses an electromagnet coil in series with the motor circuit that directly operates contacts. This is similar to a control relay but requires a rather high fault current to operate
9520-497: The public to send messages (called telegrams ) addressed to any person in the country, for a fee. Beginning in 1850, submarine telegraph cables allowed for the first rapid communication between people on different continents. The telegraph's nearly-instant transmission of messages across continents – and between continents – had widespread social and economic impacts. The electric telegraph led to Guglielmo Marconi 's invention of wireless telegraphy ,
9632-420: The receiver from the high power of the transmitter. Such relays are often used in transceivers which combine transmitter and receiver in one unit. The relay contacts are designed not to reflect any radio frequency power back toward the source, and to provide very high isolation between receiver and transmitter terminals. The characteristic impedance of the relay is matched to the transmission line impedance of
9744-480: The receiving end. The system was very stable and accurate and became accepted around the world. The next improvement was the Baudot code of 1874. French engineer Émile Baudot patented a printing telegraph in which the signals were translated automatically into typographic characters. Each character was assigned a five-bit code, mechanically interpreted from the state of five on/off switches. Operators had to maintain
9856-462: The relay is driving a large, or especially a reactive load, there may be a similar problem of surge currents around the relay output contacts. In this case a snubber circuit (a capacitor and resistor in series) across the contacts may absorb the surge. Suitably rated capacitors and the associated resistor are sold as a single packaged component for this commonplace use. If the coil is designed to be energized with alternating current (AC), some method
9968-450: The return current and one for a signal bell. When at the starting station the operator pressed a key, the corresponding pointer was deflected at the receiving station. Different positions of black and white flags on different disks gave combinations which corresponded to the letters or numbers. Pavel Schilling subsequently improved its apparatus by reducing the number of connecting wires from eight to two. On 21 October 1832, Schilling managed
10080-569: The safety design are the one defined in type B standards such as EN 13849-2 as Basic safety principles and Well-tried safety principles for machinery that applies to all machines. Force-guided contacts by themselves can not guarantee that all contacts are in the same state, however, they do guarantee, subject to no gross mechanical fault, that no contacts are in opposite states. Otherwise, a relay with several normally open (NO) contacts may stick when energized, with some contacts closed and others still slightly open, due to mechanical tolerances. Similarly,
10192-473: The same year, instead of a voltaic pile , Gauss used an induction pulse, enabling him to transmit seven letters a minute instead of two. The inventors and university did not have the funds to develop the telegraph on their own, but they received funding from Alexander von Humboldt . Carl August Steinheil in Munich was able to build a telegraph network within the city in 1835–1836. In 1838, Steinheil installed
10304-426: The sending rate. There were many experiments with moving pointers, and various electrical encodings. However, most systems were too complicated and unreliable. A successful expedient to reduce the cost per message was the development of telegraphese . The first system that did not require skilled technicians to operate was Charles Wheatstone's ABC system in 1840 in which the letters of the alphabet were arranged around
10416-402: The system, for example, 50 ohms. A contactor is a heavy-duty relay with higher current ratings, used for switching electric motors and lighting loads. Continuous current ratings for common contactors range from 10 amps to several hundred amps. High-current contacts are made with alloys containing silver . The unavoidable arcing causes the contacts to oxidize; however, silver oxide is still
10528-433: The two clicks. The message was then written out in long-hand. Royal Earl House developed and patented a letter-printing telegraph system in 1846 which employed an alphabetic keyboard for the transmitter and automatically printed the letters on paper at the receiver, and followed this up with a steam-powered version in 1852. Advocates of printing telegraphy said it would eliminate Morse operators' errors. The House machine
10640-416: The use of sound operators eliminated the need for telegraph receivers to include register and tape. Instead, the receiving instrument was developed into a "sounder", an electromagnet that was energized by a current and attracted a small iron lever. When the sounding key was opened or closed, the sounder lever struck an anvil. The Morse operator distinguished a dot and a dash by the short or long interval between
10752-441: The voltage appearing across an inductor or coil is due to a change in current which causes a change in the magnetic field within the coil, and therefore the self-induced voltage. The polarity of the voltage at every moment opposes that of the change in applied voltage, to keep the current constant. The term back electromotive force is also commonly used to refer to the voltage that occurs in electric motors where there
10864-410: Was a five-needle, six-wire system, and had the major advantage of displaying the letter being sent so operators did not need to learn a code. The insulation failed on the underground cables between Paddington and West Drayton, and when the line was extended to Slough in 1843, the system was converted to a one-needle, two-wire configuration with uninsulated wires on poles. The cost of installing wires
10976-415: Was an early needle telegraph . It had a transmitting device that consisted of a keyboard with 16 black-and-white keys. These served for switching the electric current. The receiving instrument consisted of six galvanometers with magnetic needles, suspended from silk threads . The two stations of Schilling's telegraph were connected by eight wires; six were connected with the galvanometers, one served for
11088-435: Was approved for a telegraph between the imperial palace at Peterhof and the naval base at Kronstadt . However, the project was cancelled following Schilling's death in 1837. Schilling was also one of the first to put into practice the idea of the binary system of signal transmission. His work was taken over and developed by Moritz von Jacobi who invented telegraph equipment that was used by Tsar Alexander III to connect
11200-533: Was considerably modified from the original American Morse code , and was based on a code used on Hamburg railways ( Gerke , 1848). A common code was a necessary step to allow direct telegraph connection between countries. With different codes, additional operators were required to translate and retransmit the message. In 1865, a conference in Paris adopted Gerke's code as the International Morse code and
11312-658: Was created in 1852 in Rochester, New York and eventually became the Western Union Telegraph Company . Although many countries had telegraph networks, there was no worldwide interconnection. Message by post was still the primary means of communication to countries outside Europe. Telegraphy was introduced in Central Asia during the 1870s. A continuing goal in telegraphy was to reduce the cost per message by reducing hand-work, or increasing
11424-523: Was eventually adopted. This was a two-needle system using two signal wires but displayed in a uniquely different way to other needle telegraphs. The needles made symbols similar to the Chappe optical system symbols, making it more familiar to the telegraph operators. The optical system was decommissioned starting in 1846, but not completely until 1855. In that year the Foy-Breguet system was replaced with
11536-669: Was far less limited than the momentary discharge of an electrostatic machine , which with Leyden jars were the only previously known human-made sources of electricity. Another very early experiment in electrical telegraphy was an "electrochemical telegraph" created by the German physician , anatomist and inventor Samuel Thomas von Sömmering in 1809, based on an earlier 1804 design by Spanish polymath and scientist Francisco Salva Campillo . Both their designs employed multiple wires (up to 35) to represent almost all Latin letters and numerals. Thus, messages could be conveyed electrically up to
11648-484: Was granted a patent on 4 July 1838. Davy also invented a printing telegraph which used the electric current from the telegraph signal to mark a ribbon of calico infused with potassium iodide and calcium hypochlorite . The first working telegraph was built by the English inventor Francis Ronalds in 1816 and used static electricity. At the family home on Hammersmith Mall , he set up a complete subterranean system in
11760-522: Was henceforth the international standard. The US, however, continued to use American Morse code internally for some time, hence international messages required retransmission in both directions. In the United States, the Morse/Vail telegraph was quickly deployed in the two decades following the first demonstration in 1844. The overland telegraph connected the west coast of the continent to
11872-427: Was implemented in Germany during the 1930s as a network used to communicate within the government. At the rate of 45.45 (±0.5%) baud – considered speedy at the time – up to 25 telex channels could share a single long-distance telephone channel by using voice frequency telegraphy multiplexing , making telex the least expensive method of reliable long-distance communication. Automatic teleprinter exchange service
11984-539: Was installed on the Euston to Camden Town section of Robert Stephenson 's London and Birmingham Railway in 1837 for signalling rope-hauling of locomotives. It was rejected in favour of pneumatic whistles. Cooke and Wheatstone had their first commercial success with a system installed on the Great Western Railway over the 13 miles (21 km) from Paddington station to West Drayton in 1838. This
12096-487: Was introduced into Canada by CPR Telegraphs and CN Telegraph in July 1957 and in 1958, Western Union started to build a Telex network in the United States. Back EMF Counter-electromotive force ( counter EMF , CEMF , back EMF ), is the electromotive force (EMF) manifesting as a voltage that opposes the change in current which induced it. CEMF is the EMF caused by electromagnetic induction . For example,
12208-407: Was replaced by a series of ever faster and ever smaller memory technologies. A machine tool relay is a type standardized for industrial control of machine tools , transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally open to normally closed status, easily replaceable coils, and
12320-469: Was the last great barrier to full automation. Large telegraphy providers began to develop systems that used telephone-like rotary dialling to connect teletypewriters. These resulting systems were called "Telex" (TELegraph EXchange). Telex machines first performed rotary-telephone-style pulse dialling for circuit switching , and then sent data by ITA2 . This "type A" Telex routing functionally automated message routing. The first wide-coverage Telex network
12432-426: Was ultimately more economically significant than the cost of training operators. The one-needle telegraph proved highly successful on British railways, and 15,000 sets were in use at the end of the nineteenth century; some remained in service in the 1930s. The Electric Telegraph Company , the world's first public telegraphy company, was formed in 1845 by financier John Lewis Ricardo and Cooke. Wheatstone developed
12544-533: Was used on four main American telegraph lines by 1852. The speed of the House machine was announced as 2600 words an hour. David Edward Hughes invented the printing telegraph in 1855; it used a keyboard of 26 keys for the alphabet and a spinning type wheel that determined the letter being transmitted by the length of time that had elapsed since the previous transmission. The system allowed for automatic recording on
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